Annealing

ABSTRACT

A method and apparatus for annealing an integrated ferroelectric device ( 10 ) is disclosed in which the device ( 10 ) comprises a first layer of material capable of existing in a ferroelectric state and a second layer of material defining an integrated circuit below the first layer such as a microbridge thermal detector. The method comprises producing a pulse of energy, extending the pulse temporally using a pulse extender ( 200 ) and illuminating the first layer with the extended pulse. The duration and wavelength and fluence of the extended pulse are selected so that the material of the first layer is annealed into a ferroelectric state without exceeding the temperature budget of the integrated circuit. Application of the method in heating other articles which comprise a layer to be heated and a temperature sensitive layer is also disclosed. By extending the temporal width of the pulse, energy is supplied at a rate which ensures a more even heating of the first layer without damaging the temperature sensitive layer over time.

This application is the US national phase of international applicationPCT/GB00/00753, filed in English on 3 Mar. 2000, which designated theUS. PCT/GB00/00753 claims priority to GB Application No. 9905098.1 filed6 Mar. 1999. The entire contents of these applications are incorporatedherein by reference.

This invention relates to improvements in annealing, and in particularto a method and apparatus for annealing ferroelectric thin filmmaterials.

There has been a considerable amount of research into the development ofdevices which utilise the thermal properties of ferroelectric materials.One example is the development of infrared imaging cameras based ontwo-dimensional arrays of ferroelectric thermal detectors which areattractive due to their near ambient temperature operation. Thermaldetectors used for infra-red imaging rely on the temperature change ofthe sensing material due to absorption of infra-red radiation. Withferroelectric materials this radiation causes a change in the electricalpolarisation of the material which enables the magnitude of the changein temperature to be detected.

In order to reduce the size of the detectors, combined integratedferroelectric devices have been developed in which the ferroelectricmaterial is combined with the electronic read out circuitry in a singledevice. Typically, these devices comprise layered structures with a thinlayer of ferroelectric sputtered or spin coated or otherwise depositedonto or above one or more base layers. Other examples of such integratedferroelectric devices are thin film piezoelectric actuators andferroelectric random access memories (FeRAM).

The combination of the ferroelectric material with the active circuitryin one package produces a more compact device than the provision of aseparate read out circuit and improves yield, reduces cost and improvesperformance. However, a fundamental problem with such devices is theneed to deposit the ferroelectric material within a thermal budget thatis compatible with the integrated circuitry not being damaged ordestroyed by elevated temperatures. It is widely recognised thatexposure of an integrated circuit to temperatures above 450° C. is aconstraint on the processing of chips/materials with IC content, andthis conflicts with the growth requirements of many ferroelectriclayers.

A particularly important family of ferroelectric materials in use andunder investigation for IR detector, actuator or FeRAM applications isthe perovskites. This family include materials such as lead scandiumtantalate (PST), lead zirconate titanate (PZT), barium strontiumtitanate (BST), lead titanate (PT) and others. For use as aferroelectric the material layer must be in the perovskite phase. It caneither be deposited directly into that phase at an elevated temperatureor at a lower temperature which is then subsequently annealed into theferroelectric perovskite phase. Layers deposited at low temperatures aregenerally in an amorphous, pyrochlore or other phase which is incapableof exhibiting ferroelectricity. For PST, for example, the material mustbe deposited at temperatures in excess of 450° C. to enter theperovskite phase. Direct depositing of these materials in a perovskitephase is therefore incompatible with the temperature budgets ofintegrated circuitry.

One known way of providing a layer of ferroelectric material in theperovskite phase without damaging an ROIC provided on a base layer is todeposit the material in a non-ferroelectric state at a low temperature(say less than 450° C.). The material may then be annealed using a laserto heat the layer sufficiently to convert the material into itsperovskite phase.

In order to understand the effect of laser annealing, consider thestructure of a typical uncooled microbridge type IR detector. From thetop surface downwards prior to the electrode deposition, the layers ofmaterial are as given in Table 1.

TABLE 1 Material Thickness Purpose PST 1 μm Ferroelectric Pt 1000ÅBottom electrode Ti 50Å Adhesion layer SiO2 1000Å Barrier layer Sac1–2.5 μm Sacrificial layer SiO2 0.8 μm Passivation/barrier layer ROICActive circuitry Si 500 μm Substrate wafer

In order to heat the PST layer sufficiently without damaging the ROIC(Read Out Integrated Circuitry) layer, the laser wavelength must bechosen so that strong absorption occurs in the PST layer. The temporalwidth of the pulse must also be kept sufficiently short that the heatdiffusion length is small enough to prevent the induced heat wave frompenetrating through the layers to the ROIC layer. For relatively thinlayers up to, say, 1000 μm these criteria are satisfied by commerciallyavailable excimer lasers. These lasers operate in the ultraviolet (UV)and have short pulse lengths of around 25 ns. Pulses are delivered tothe layer as either a single shot or at a slow repetition rate of 100'sof Hz or thereabouts. The measured reflectively of low temperaturedeposited (non-perovskite) PST at the wavelength of a typical commercialexcimer laser of 248 nm is 21%. This indicates strong absorption, whilstthe absorption length at this wavelength, calculated from experimentaldata, is 19 nm indicating strong surface absorption.

To generate sufficient temperature at the bottom of a thick layer of PST(greater than 1000 Å) requires relatively high energy density excimerlasers. Increasing the power increases the surface temperature. Thisplaces a limit on the maximum possible thickness of PST which can beannealed at around 1000 Å due to extreme surface heating which can causesurface damage, poor crystallisation and crystal quality, poor filmphysical integrity and loss of stoichiometry due to evaporation ofvolatile components. As a result, the use of such a laser isunsatisfactory for layers above, say 2000 Å (and more so for eventhicker layers), with extremely high temperatures being generated at thesurface of the layer and high thermal gradients being generated in thePST due to the short absorption and diffusion lengths resulting from theshort pulse duration.

An object of the present invention is to overcome or alleviate theproblems involved in the laser annealing of relatively thick layers offerroelectric materials posed by the constraints of commerciallyavailable lasers.

In accordance with a first aspect, the invention provides a method ofproducing an integrated ferroelectric device comprising a first layer ofmaterial capable of existing in a ferroelectric state and a second layerof material defining an integrated circuit, the method comprising thesteps of:

-   producing a pulse of energy having a first temporal width;-   extending the temporal width of said pulse by passing it through a    temporal extender to produce a processed pulse having a greater    temporal width; and-   illuminating the first layer with said processed pulse to convert    some or all of the material in the first layer from a    non-ferroelectric state into a phase capable of exhibiting    ferroelectricity or otherwise improving the quality of the material    of the first layer without exceeding the temperature budget of the    integrated circuit of said second layer.

The method may further comprise generating a number of such processedpulses and sequentially illuminating the device with said pulses.

Preferably, the material of the first layer comprises a low gradedeposited perovskite and the method improves the quality (i.e. greatercrystal order and/or crystal size) of the perovskite material.Alternatively, the material may be deposited substantially in anon-perovskite phase and the method converts some or all of the materialinto the perovskite phase.

Preferably, the pulse of energy comprises a pulse of energy producedusing a laser. More than one such pulse may be produced, with each pulsebeing temporally extended. The first layer may be sequentiallyilluminated by a number of such processed pulses.

By providing a temporal extender, it is possible to deliver the laserenergy to the first layer at a slower rate than is possible with anon-extended laser pulse using commercially available laser sources.This increases the diffusion length in the material ensuring more evenheating throughout the layer and reduced surface temperatures at theface of the first layer nearest the laser source.

The laser pulse may be produced using an excimer laser. It may have awavelength in the ultraviolet of around 248 nm (for a KrF excimerlaser). Alternatively, the pulse may be produced using a CO₂ laser.

The pulse produced by the laser may have a temporal length ofsubstantially 10 ns, or 20 ns or perhaps substantially 25 ns, or evenbeyond or any value within a range of values limited by one or more ofthe preceding values. This represents the limit for current commerciallyavailable excimer lasers.

The temporal extender may increase the temporal length of the pulse toproduce a processed pulse with a temporal length of approximately 300ns, or between substantially 300 ns and 400 ns, or perhaps longer. Forinstance, the processed pulse may have a temporal length that is anorder of magnitude greater than the unprocessed pulse.

The extended pulse may comprise more than one sub-pulse, each sub-pulsecorresponding to a pulse action of the extender. These may be separatedby a known time interval to produce a sequence of closely spacedsub-pulses defining the processed pulse. The temporal width of eachsubpulse may correspond to the temporal width of the unprocessed pulse.They may be separated by, say, substantially 25 ns or substantially 30ns or substantially 50 ns or less or more or any range bounded by one ofthe values. In the case of ten sub pulses, an processed pulse with atemporal width of about 400 ns is produced.

The processed pulse may, therefore, comprise two, three, four, orperhaps ten or more sub-pulses which are temporally spaced closetogether to form a processed pulse. By close we mean that the spacingbetween sub-pulses may be less than the width of each sub-pulse orperhaps equal to the sub-pulse width, or larger than the sub pulsewidth.

Each sub-pulse may be produced by partial reflection of the unprocessedpulse within the temporal extender.

The method may comprise producing a processed pulse having a fluence andtemporal width that is compatible with the properties of the material ofthe first layer such that the temperature throughout the layer (or overa substantial depth of the first layer) exceeds a predetermined annealtemperature whilst the temperature of the second layer is within thetemperature budget of the circuitry.

In one especially useful embodiment, the processed pulse properties(including fluence, temporal width and wavelength) may be selected sothat the whole of the first layer exceeds the transition temperature forplacing the material into a ferroelectric perovskite phase. This may begreater than 450° C. At the same time, the peak temperature in thesecond layer may be kept lower than 450° C.

The first layer may comprise a layer of PST (or other material) ofthickness of substantially 0.1 μm, or perhaps substantially 1 μm, orperhaps substantially 0.8 μm or substantially 1.2 μm or any valuetherebetween.

The first layer may comprise the top layer of the device. Alternatively,it may have other layers provided both above and below it defining asandwich like structure. The integrated circuitry may be provided belowthe first layer.

In a refinement of the method, at least one additional layer may beprovided above the first layer (i.e. on the opposite side to the secondlayer).

Two different sources of energy may be used, each source producing arespective pulse and each respective pulse being extended by a pulseextender to produce a processed pulse. The first layer may then beilluminated by both processed pulses. This may be either substantiallysimultaneously or sequentially.

The different sources of energy may comprise two different lasers, eachgenerating a pulse of a different wavelength. For instance, one sourcemay comprise a KrF laser or other type of excimer laser, whilst theother source comprises a carbon dioxide (CO₂) laser.

In one arrangement, the method may include providing a metallic layerbetween the first layer and the second layer and illuminating the firstlayer with two different processed pulses. A CO₂ laser and a KrF lasermay be used whereby two effects arise. Firstly, the first layer isheated from the top down by the KrF laser pulse. Secondly, the firstlayer is heated up from the bottom upwards due to heating of themetallic layer when excited by the CO₂ laser pulse. This effectivelyheats the layer from both sides.

The fluence of the pulse may be substantially be 0.05 J/cm², 0.1 J/cm²,0.2 J/cm² or perhaps a higher value or a lower value. It may be selectedto be any value within a range bounded at its upper limit and/or lowerlimit by one or more of these values. For example, it may be in therange 0.05–0.1 J/cm² or 0.1–0.2 J/cm² or 0.05–0.1 J/cm².

The temporal width and fluence of the processed pulse may also be chosento match the thickness and properties of the first layer materials sothat the surface temperature at the first layer (i.e. the surfaceexposed to radiation) does not exceed a predetermined maximumtemperature. Using a suitable processed pulse width on a 1 μm layer, itis possible to keep the surface temperature below the perovskite meltingtemperature of approximately 1500° C. (depending on the material used).

Of course, it will be readily appreciated that instead of commencingwith a short pulse having a first temporal duration, and extending thepulse, a longer initial pulse could be employed by using a bespoke laserdevice. However, this would prove more costly and so is not preferred.We may wish to seek protection for such a method in which a non-extendedpulse is used.

In a refinement, the first layer may be illuminated with the processedpulse whilst the ambient temperature of the device is maintained higherthan room temperature. An ambient temperature in the range 100° C.–450°C., or 200° C.–450° C., or any other range between the limits 100° C.and 500° C. approximately could be used. An ambient temperature of 300°C. is preferred. This means that it is possible to use a lower energylaser pulse source it has less work to do to raise the temperaturesabove the phase transition temperature. In a further refinement, thelaser light could illuminate the substrate during deposition of thefirst layer.

It will also be appreciated that there may be a considerable time delaybetween depositing the layer (or layers) forming the device and theannealing steps. For instance, the device layers may be deposited in onefactory or room within a factory before moving to another room orfactory to be annealed. Indeed, the method may find application inannealing any device having first and second layers at any time in itslife or before or after use.

The first layer of material may be deposited at a temperature below thatneeded to form perovskite and may be deposited substantially wholly asnon-perovskite phase. For instance, for PST a non-perovskite layer wouldbe produced by depositing at below 300° C. (to produce an amorphousmaterial) or between 300° C. or at 500° C. to produce a pyrochlorematerial. The higher the temperature used to deposit, the more likelythat some of the material will exist in the perovskite phase beforeannealing. Of course, the temperature used to deposit the material mustnot exceed the temperature budget of the second layer.

By temperature budget, the skilled man will of course appreciate that wemean the maximum temperature that the second layer can be heated towithout causing unacceptable damage or degradation to the second layer.

In accordance with a second aspect, the invention provides an apparatusfor producing an integrated ferroelectric device, said device comprisingat least a first layer of material capable of existing in a perovskitephase and second layer of material defining an integrated circuit, theapparatus comprising:

-   pulse generating means adapted to generate a pulse of energy having    a first temporal width;-   pulse extending means adapted to extend the temporal pulse width of    said pulse to provide a processed pulse of greater temporal width;-   and guide means adapted to guide said processed pulse of energy onto    said first layer whereby some or all of the material in the first    layer is converted from a non-ferroelectric state into a    ferroelectric state or to otherwise improve the quality of the    material of the first layer without exceeding the temperature budget    of the integrated circuit of said second layer.

The apparatus may further comprise depositing means for depositing saidfirst layer of material above said second layer in which some or all ofsaid first layer is in a non-perovskite phase.

The pulse generating means may, for example, comprise a laser such as anexcimer laser, such as a krypton fluoride (KrF) laser. Alternatively, itmay comprise a carbon dioxide (CO₂) laser. The laser may have awavelength in the ultraviolet spectrum, for instance 248 nm. An exampleof a suitable laser is the Lambda Physik LPX210i Krf excimer laser.

The depositing means may be adapted to deposit a first layer of materialabove the second layer after one or more intermediate layers aredeposited onto the second layer. One of these intermediate layers maycomprise a sacrificial layer that is subsequently removed to leave aspace between the first and second layers to form a microbridge. Byproviding electrical contacts between the integrated circuitry of thesecond layer and the first layer the device may act as an infraredthermal detector.

Removal means may therefore be provided for removing the sacrificiallayer. The layer be removed before or after the first layer is annealed.

The device may comprise a thermal detector such as a pyroelectric ordielectric botometer type infrared detector. This may comprise an arrayof device defining a thermal imaging camera, perhaps an uncooled array.Alternatively, it may comprise a piezoelectric actuator or perhaps aferroelectric random access memory. A number of devices may be providedin an array on a single wafer. Of course, in a modification it will bewithin the scope of protection sought to provide an apparatus forannealing any device which includes a first layer that is to be annealed(perhaps not ferroelectric) and a second layer which is sensitive tooverheating.

Where an array of devices are provided, the processed pulse mayilluminate more than one or preferably all of the devices in the arraysimultaneously. Alternatively, the processed pulse may be appliedsequentially to the devices by scanning a laser beam made up of a numberof processed pulses across the array of devices. In another arrangement,the laser may be fixed whilst the array of devices moves relative to thelaser using one or more translation stages.

The apparatus may further include means for raising the ambienttemperature of the integrated device during annealing. This may comprisea heating element upon which the device is placed.

The apparatus may further comprise means for evacuating the air fromaround the device during annealing. For example, a vacuum chamber may beprovided with the device being placed in the vacuum chamber. An inletport may be provided whereby the chamber can be filled with one or moregases, such as oxygen, during the annealing.

The pulse extender may be adapted to increase the temporal pulse widthof the first pulse by substantially two times or four times, orsubstantially ten times or more than ten times or any valuetherebetween. In one arrangement, the pulse extender may be adapted toproduce a processed pulse that comprises a number of sub-pulses, eachsub pulse corresponding to the first pulse. This may be achieved usingpartial multiple reflections of the first pulse. A suitable pulseextender can be obtained from Exitech Limited, Hanborough Park, LongHanborough, Oxford, OX8 8LH.

In accordance with a third aspect, the invention provides an integratedferroelectric device comprising at least a first layer of ferroelectricmaterial and a second layer comprising an integrated circuit, in whichsaid first layer is transformed into a perovskite phase using a pulse ofenergy from a laser that has been temporally extended.

The first layer may comprise a material selected from the class ofmaterial which can exist in a perovskite phase, such as PST, leadzirconate titanate (PZT), barium strontium titanate (BST), lead titanate(PT) and others.

The second layer may comprise silicon, silicon oxide and the requisitemetallisation and implant doping layers to define the integratedcircuit. Of course, other materials are possible.

The device may comprise a microbridge. This may form a part of animaging device to detect incident radiation. An array of devices may beprovided, for example to produce a two-dimensional image of a scene. Inthis case, the first layer may be spaced from the second layer to definea bridge overlapping the second layer.

The upper surface of the bridge may have a metal coating. The lowersurface of the bridge may have a metal coating as well as or instead ofthe upper layer. The metal coating on the lower surface of the bridgemay comprise a platinum layer overlapping a titanium layer. The metalcoating on the upper surface may comprise a titanium layer.

The integrated circuit of the second layer may comprise a read outintegrated circuit (ROIC). It may include an amplifier adapted toamplify signals from the first layer.

A barrier layer, such as silicon dioxide may be provided on the lowersurface of the bridge. This may overlap any metallic layer that ispresent. The second layer may also be provided with a barrier layer.Again, this may be silicon dioxide for example. The barrier layersprevent sacrificial material which defines the gap between the first andsecond layers from reacting with the layers during manufacture of thedevice. Of course, in the finished device, substantially all of thesacrificial material may well have been removed using a suitable etch. Athermal barrier layer, e.g. a layer of SiO₂ having a low thermaldiffusivity may be provided below the first layer in the gap to improvethe downward flow of heat away from the first layer.

Of course, the device does not need to comprise a microbridge. It may,for example, comprise an alternate form of infrared sensing device. Itmay be a thin film piezoelectric actuator or a ferroelectric randomaccess memory (FeRAM) or a dynamic random access memory (DRAM).

Most preferably the thickness of the first layer of ferroelectricmaterial is greater than 2000 Å, or greater than 5000 Å, or up to 1 μm.Substantially all of the material throughout the thickness of the layermay exist in the perovskite phase which is annealed using the extendedlaser pulse.

In accordance with yet a further aspect, the invention provides a methodof preferentially heating a first layer of material to a firsttemperature without heating a second layer of a material provided belowsaid first layer to said first temperature by illuminating the firstlayer with a temporally extended pulse of radiation from a laser source.

By temporally extending a pulse, the surface temperature of the firstlayer when illuminated with sufficient pulse energy to heat the wholelayer up to or above the first temperature is lower than for acorresponding un-extended pulse where the energy is supplied in ashorter period of time.

Of course, in an alternative the first layer may be heated to such anextent that the uppermost surface layer exceeds its melting point or isotherwise damaged. After annealing, this damaged (unwanted) layer couldbe milled away or otherwise removed.

The first layer may comprise a material which is capable of existing ina ferroelectric state. The second layer may comprise a silicon base uponwhich an integrated circuit is formed. It is generally accepted thatsuch circuits will be damaged if heated in excess of 450° C., whilstmost ferroelectric materials need to be heated in excess of 450° C. toproduce a high grade ferroelectric material. This can be achieved usingthe method of the present invention for layers of ferroelectric materialup to at least 1 μm in thickness.

Preferably, the pulse width is selected so that it is sufficiently longto ensure that the heat at the surface of the first layer diffuses awayat a rate that keeps the surface temperature below melting, yetsufficiently long that the surface temperature of the second layer doesnot exceed the first temperature. This will depend on the properties ofthe first layer and the wavelength of the pulse.

In accordance with a still further aspect, the invention provides amethod of heat treating a treatment layer of an article comprising saidtreatment layer and one or more further layers, the method comprisingthe steps of illuminating the article with a laser pulse which istemporally extended by a temporal extender, the temporal width andfluence of the pulse being selected so that the treatment layer israised to a treatment temperature T whilst one or more of the furtherlayers are kept substantially below the treatment temperature T.

Preferably the treatment layer extends from a surface of the articledownwards with one or more further layers below the treatment layer.

Alternatively, the treatment layer may be provided between one or moreother layers.

The laser pulse may be temporally extended by partial reflection of alaser beam.

The heat treatment may comprise an annealing process. This may beadapted to convert the state of some or all of the material of thetreated layer from its untreated state to a different treated state. Anexample is conversion of a ferroelectric material from a non-perovskiteto a perovskite state, or from low grade perovskite to higher gradeperovskite. Other treatments include heating to relieve locked instresses within the first layer, or simply, to cause chemical change inthe material of the layer or some other physical change of state.

In accordance with a still further aspect, the invention providesapparatus adapted to heat treat a treatment layer of an articlecomprising a treatment layer and one or more further layers, comprisinga laser adapted to produce a laser pulse, a temporal extender adapted toextend the temporal width of the laser pulse, and means for guiding thetemporally extended pulse onto the article, whereby the laser pulse isadapted to raise the temperature of that treatment layer above atreatment temperature T whilst one or more of the further layers arekept substantially below the treatment temperature.

Looked at one way, an object of the invention in at least onearrangement is to provide a more uniform heating of the material in thefirst layer than can be achieved using standard commercially availablelaser devices. Alternatively, it may be considered as a technique andapparatus for reducing the surface temperature at the first layer byproviding the required heat energy at a slower rate using an extendedpulse. In another aspect, the invention ensures that heat is kept awayfrom unwanted areas of the article by providing a suitable durationextended pulse which provides more controlled and accurate heating ofthe layer of material.

There will now be described, by way of example only, one embodiment ofthe present invention with reference to the accompanying drawings ofwhich:

FIG. 1 is a representation of an integrated uncooled IR detector havinga ferroelectric layer that is annealed in accordance with the presentinvention;

FIG. 2 is a plot of the measured temporal pulse shape for a prior artLambda Physik LPX210i excimer laser;

FIG. 3 illustrates a set of predicted prior art temperature profilesthrough 1 micron of PST due to exposure to a non-extended laser pulse ofwidth 25 nm and a fluence of 0.1 J/cm²;

FIG. 4 illustrates the predicted prior art temperature profiles throughthe depth of the PST layer stack during and after exposure to anon-extended laser pulse with fluence 0.1 J/cm²;

FIG. 5 illustrates the measured temporal pulse shape for a x 10pulse-extension action with a 35 ns interpulse delay using an excimerlaser as in accordance with the present invention;

FIG. 6 shows predicted temperature profiles for a 1 micron layer of PSTwhen exposed to an extended laser pulse with fluence 0.2 J/cm²;

FIG. 7 shows predicted temperature profiles through the depth of the PSTlayer stack during and after exposure to an extended laser pulse withfluence 0.2 J/cm²;

FIG. 8 is an XRD θ-2θ scan of as deposited, unannealed, layer of sol-gelPZT;

FIG. 9 is an XRD θ-2θ scan of pulse-extended laser annealed sol-gel PZT;and

FIG. 10 is a cross-section illustrating a typical microbridge deviceannealed in accordance with the invention; and

FIG. 11 illustrates an apparatus of one embodiment of the invention.

In order to quantify the improvements that are attainable using a methodand apparatus in accordance with the present invention a model of thetemperature profiles throughout a typical infra-red sensing device wasdeveloped and experimental data was obtained using both a prior artsingle pulse laser and a pulse-extended laser in accordance with theinvention.

The diffusion equation for heating of an arbitrary material using laserradiation has been solved. Assuming the layer stack and the irradiatinglaser beam to be uniform in the x-y plane, the later due to the use of abeam homogeniser, then the equation can be expressed in auni-dimensional form as:$\frac{\partial T}{\partial t} = {{\frac{\alpha}{\rho\; c_{p}}{I\left( {z,t} \right)}} + {\sigma\;{ɛ\left( {T^{4} - T_{a}^{4}} \right)}} + {\frac{1}{\rho\; c_{p}}\frac{\partial}{\partial z}\left( {k\frac{\partial T}{\partial z}} \right)}}$where I (z, t) is the power density of the laser light at a depth z andat a time t, T is the temperature in the absorbing medium, T_(a) is theambient temperature of the annealing chamber and ε, ρ, Cp, k and α arethe emissivity, density, specific heat, thermal conductivity andabsorption coefficient respectively. By inputting the thermal propertiesof a ferroelectric material such as PST and the temporal form of thelaser pulse applied to the material, the temperature distributionthroughout the material can be estimated. It should, however, be notedthat the effect of latent heat due to the formation of the perovskitephase was not taken into account, but this does not effect theinvention.

The structure of a typical sensor is shown in cross section in FIG. 10and in plan in FIG. 1 of the accompanying drawings (and in Table 1 onpage 3).

The sensor comprises a microbridge 10 as shown in FIG. 1 which has amain detector area 12 which forms a single pixel in an array of pixelsin an imaging device. The main detector area 12 comprises a bridge 14 ofsensing material that is responsive to incident radiation, typically aferroelectric material such as Lead Scandium Tantalate. The microbridgearea is typically 50 microns by 50 microns. The leg width is around 5microns, and the length of the leg around 30 microns. The sensingsensing material is provided with electrically conducting coatings onboth the lower and upper faces. The coating on the upper face ispatterned to be only on the upper bridge body area. The coating on thelower face is patterned in the same shape as the sensing material, andthus is continuous down the microbridge legs. An electricaldiscontinuity (not shown) in the lower coating separates the bridge 14into two areas. A substrate or base layer (not shown) of silicon isprovided, and the bridge 14 is supported away from the silicon baselayer by a pair of legs 18 which slope down to contact the main detectorarea 12 at diagonally-opposed corners 20 and 22. Feet 24 and 26 of thelegs contact the silicon base layer.

FIG. 10 shows a cross-section view of the microbridge in FIG. 2 taken soas to pass through the two feet of the microbridge (not to scale). Inthe arrangement of FIG. 10, there is a silicon base layer or substrate Awhich has a depth of, typically, 300 to 500 microns, a silicon dioxideinsulating layer B extending above the silicon layer with a depth ofabout 0.5 microns, a space G (filled with sacrificial material duringthe manufacture of the device) with an average depth of 1 to 2 microns,a titanium layer and a platinum layer D (titanium of the order of 100 Å,platinum layer of the order of 1,000 Å), a ferroelectric layer E, inthis example of Lead Scandium Tantalate having a depth/thickness ofabout 1 micron, and a titanium layer F having a depth of about 100 to200 Å.

The upper and lower surfaces of the ferroelectric material have metalliccoatings. The metal coating on the lower surface comprises a platinumlayer overlaying in titanium layer. The metal coating on the uppersurface comprises a titanium layer. The distance between the conductinglayers D and F of FIG. 2, referenced H, when combined with therefractive index of the sensing material, results in an optical pathequal to one quarter of the wavelength of the radiation to which themicrobridge is to be sensitive. Thus, the microbridge is tuned forabsorption to a particular wavelength by the thickness of the thermallysensitive layer. For 10 micron wavelength radiation this translates toaround 1 micron physical thickness for most ferroelectric ceramicmaterials, and preferably for lead scandium tantalate. For optimumabsorption, the titanium coating on the upper surface has a sheetresistance matched to free-space i.e. 377 ohms per square, and theplatinum coating on the lower surface has a high infra-red reflectancei.e. is over 100 nanometers thickness. The lower coating has adiscontinuity, I, (to give, effectively, two parallel-plate capacitorplates connected back-to-back in series). This allows the lower coatingto be used to give two contacts to the sensing material and allows theupper coating to be electrically floating.

Conductive metal interconnect tracks, C, are provided on the siliconbase layer, to connect the signals from the microbridge to the read-outelectrodes. The silicon base layer is coated with an insulating layer,B, in this case silicon dioxide, to electrically insulate theinterconnect tracks. The two halves of the lower coating on the sensingmaterial are connected each to a separate interconnected track on thesilicon base layer. In an imaging device where readout electronics arein the base silicon layer, then the interconnect tracks will bepatterned with the read-out electronics i.e. under the silicon dioxideor other similar passivation layer, B, and the contacts to themicrobridge will be through a via in the passivation layer.

Shown in FIGS. 3 and 4 are the temperature distributions throughout thelayers of the device of FIG. 10 due to a 25 cm wide laser pulse from anexcimer laser with a fluence of 0.1 j/cm². The temperature distributionwas modelled in simulation by ten discrete 0.1 μm thick layers, the meantemperatures of which are shown as a function of depth in FIG. 4. Thesurface temperature is very high as well as the temperature gradientthrough the PST layer. FIG. 4 shows the temperature distribution withdistance from the irradiated surface. Again, high temperature gradientsand excessive surface heating are apparent.

In order to solve the problem of the high surface temperature, anapparatus in accordance with an aspect of the invention has beenproposed. This is illustrated in FIG. 11. It comprises a commercialexcimer laser 100 controlled by a computer 101 which produces a lightpulse with a pulse duration at full width at half maximum (fwhm) of 25ns. The output of the laser 100 is passed through a variable attenuator150 and a temporal extender 200 which effectively increases the pulseduration and hence the diffusion length. The extended laser pulse isthen passed through an anamorphic telescope 210 and a beam homogeniser220 to a vacuum chamber 300.

The vacuum chamber 300 incorporates a heating plate 301 upon which awafer 302 containing an unprocessed sensor is placed behind a UVtransparent window 303. The heat raises the ambient temperature of thewafer defining the sensor, and the pulsed laser beam anneals the PSTlayer into a layer in the perovskite phase. A vacuum pump 304 isprovided for evacuating the chamber whilst an inlet 305 allows a processgas to be introduced to the chamber.

A measured temporal profile from the apparatus is shown in FIG. 5. Tensub-pulses can be seen corresponding to the main pulse extension actionand then decaying subsidiary pulses which correspond to imperfections inthe system. The pulse extender has increased the pulse duration from afull width at half maximum of 25 ns to one of 350 ns.

Assuming the pulse to have a fluence of 0.1 J/cm² (as for the unextendedpulse) the effect of the pulse on the layers is shown in FIG. 6 for themean layer temperatures and in FIG. 7 for the temperature distributionwith distance from the irradiated surface at various times. Again, a 1μm layer of PST modelled as ten 0.1 μm layers was assumed.

The effect of the pulse extender is to effectively slow down the rate ofenergy delivered to the surface thereby giving the heat generated moretime to diffuse away. The surface temperatures are correspondingly lowerthan for the non-extended pulse with lower thermal gradients, and hencean average higher temperatures, within the whole of the PST layer.Clearly, the pulse length is still sufficiently short to prevent thetemperature of the topmost surface of the silicon water—where the activecircuitry resides—from rising more than a few degrees above the ambienttemperature of the wafer as a whole.

Initial studies using apparatus based on the arrangement shown in FIG.11 with sol-gel deposited PZT have shown that the technique is capableof crystallising amorphous as-deposited material into the requiredferroelectric perovskite phase without damaging the underlying ROIC.

FIG. 8 shows an x-ray diffraction (XRD) θ-2θ scan of the material asdeposited. The reflections present which are visible in the Figure aredue to the platinum underlining the PZT and an intermetalic which formsdue to a reaction between the platinum in the substrate and the lead inthe as-deposited PZT.

FIG. 9 shows a similar XRD θ-2θ scan of the pulse-extended excimer laserannealed material with 10⁴ pulses at a fluence of 80 mJ/cm² with thesubstrate heated to an ambient temperature at 300° C. in an oxygenfilled chamber. Reflections from perovskite PZT can be clearly observedshowing that the material has been crystallised into the correct phase.

1. A method of producing an integrated ferroelectric device comprising afirst layer of material capable of existing in a ferroelectric state anda second layer of material defining an integrated circuit, said methodcomprising the steps of: producing a pulse of energy having a firsttemporal width; extending said first temporal width of said pulse bypassing said pulse through a temporal extender to produce a processedpulse having a greater temporal width than said first temporal width;and illuminating said first layer with said processed pulse to convertsome or all of said material in said first layer from anon-ferroelectric state into a phase capable of exhibitingferroelectricity or otherwise improving the quality of said material ofsaid first layer without exceeding the temperature budget of saidintegrated circuit of said second layer.
 2. A method to according claim1 wherein said method further comprises generating more than one of saidprocessed pulses and sequentially illuminating said first layer withsaid processed pulses.
 3. A method according to claim 1 wherein saidmaterial of said first layer comprises a low grade deposited perovskiteand said method improves the quality of said perovskite material.
 4. Amethod according to claim 1 wherein said first layer includes materialdeposited in a non-perovskite phase and said method converts some or allof the material into said perovskite phase.
 5. A method according toclaim 1 wherein said pulse of energy comprises a pulse of energyproduced using a laser.
 6. A method according to claim 5 wherein saidpulse produced by said laser has a temporal length between 10 ns and 25ns.
 7. A method according to claim 1 wherein said temporal extenderincreases the temporal length of said pulse to produce a processed pulsewith a temporal length of approximately 300 ns, or between substantially300 ns and 400 ns, or longer.
 8. A method according to claim 1 whereinsaid processed pulse comprises more than one sub-pulse, each of saidsub-pulses corresponding to a pulse action of said extender.
 9. A methodaccording to claim 1 wherein said processed pulse has a fluence andtemporal width that is compatible with properties of said material ofsaid first layer such that the temperature throughout said layer (orover a substantial depth of said first layer) exceeds a predeterminedanneal temperature whilst the temperature of said second layer is withinsaid temperature budget of said circuitry.
 10. A method according toclaim 1 wherein said first layer comprises a top layer of said device.11. A method according to claim 1 wherein two different sources ofenergy are produced, each source producing a respective pulse and atleast one of said respective pulses being extended by a pulse extenderto produce a processed pulse, and wherein said first layer isilluminated by both pulses.
 12. A method according to claim 11 whereinsaid layer is illuminated by both pulses substantially simultaneously.13. A method according to claim 11 which further comprises providing ametallic layer between said first layer and said second layer andilluminating said first layer with the two different processed pulses.14. A method according to claim 1 wherein said first layer isilluminated with said processed pulse whilst said ambient temperature ofsaid device is maintained higher than room temperature.
 15. An apparatusfor producing an integrated ferroelectric device from an initiallynon-ferroelectric device, said initially non-ferroelectric devicecomprising at least a first layer of non-perovskite phase materialhaving a thickness greater than 2000 Å and second layer of materialdefining an integrated circuit, said apparatus comprising: pulsegenerating means for generating a pulse of energy having a firsttemporal width; pulse extending means for extending said first temporalpulse width of said pulse to provide a processed pulse of greatertemporal width; and guide means for guiding said processed pulse ofenergy onto said first layer, said pulse generating means and said pulseextending means providing sufficient energy to anneal at least a portionof said non-perovskite phase material into perovskite phase material.16. Apparatus according to claim 15 which further comprises depositingmeans for depositing said first layer of material above said secondlayer in which some or all of said first layer is in a non-perovskitephase.
 17. Apparatus according to claim 16 wherein said depositing meansis adapted to deposit a first layer of material above said second layerafter one or more intermediate layers are deposited onto said secondlayer.
 18. Apparatus according to claim 17 wherein one of saidintermediate layers comprises a sacrificial layer.
 19. Apparatusaccording to claim 15 wherein said pulse generating means comprises alaser.
 20. Apparatus according to claim 19 wherein said laser has awavelength in the ultraviolet spectrum.
 21. Apparatus according to claim15 wherein said pulse extender is adapted to increase said temporalpulse width of said first pulse by substantially at least two times. 22.Apparatus according to claim 15 wherein said pulse extender is adaptedto produce a processed pulse that comprises a plurality of sub-pulses.23. An apparatus for producing an integrated ferroelectric device, saiddevice comprising at least a first layer of material capable of existingin a perovskite phase and second layer of material defining anintegrated circuit, said apparatus comprising: pulse generating meansfor generating a pulse of energy having a first temporal width; pulseextending means for extending said first temporal pulse width of saidpulse to provide a processed pulse of greater temporal width; and guidemeans for guiding said processed pulse of energy onto said first layer;and depositing means for depositing said first layer of material abovesaid second layer in which some or all of said first layer is in anon-perovskite phase.